2bit_local.cc revision 11429:cf5af0cc3be4 
1/*
2 * Copyright (c) 2004-2006 The Regents of The University of Michigan
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are
7 * met: redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer;
9 * redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution;
12 * neither the name of the copyright holders nor the names of its
13 * contributors may be used to endorse or promote products derived from
14 * this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 *
28 * Authors: Kevin Lim
29 */
30
31#include "base/intmath.hh"
32#include "base/misc.hh"
33#include "base/trace.hh"
34#include "cpu/pred/2bit_local.hh"
35#include "debug/Fetch.hh"
36
37LocalBP::LocalBP(const LocalBPParams *params)
38    : BPredUnit(params),
39      localPredictorSize(params->localPredictorSize),
40      localCtrBits(params->localCtrBits)
41{
42    if (!isPowerOf2(localPredictorSize)) {
43        fatal("Invalid local predictor size!\n");
44    }
45
46    localPredictorSets = localPredictorSize / localCtrBits;
47
48    if (!isPowerOf2(localPredictorSets)) {
49        fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
50    }
51
52    // Setup the index mask.
53    indexMask = localPredictorSets - 1;
54
55    DPRINTF(Fetch, "index mask: %#x\n", indexMask);
56
57    // Setup the array of counters for the local predictor.
58    localCtrs.resize(localPredictorSets);
59
60    for (unsigned i = 0; i < localPredictorSets; ++i)
61        localCtrs[i].setBits(localCtrBits);
62
63    DPRINTF(Fetch, "local predictor size: %i\n",
64            localPredictorSize);
65
66    DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
67
68    DPRINTF(Fetch, "instruction shift amount: %i\n",
69            instShiftAmt);
70}
71
72void
73LocalBP::reset()
74{
75    for (unsigned i = 0; i < localPredictorSets; ++i) {
76        localCtrs[i].reset();
77    }
78}
79
80void
81LocalBP::btbUpdate(Addr branch_addr, void * &bp_history)
82{
83// Place holder for a function that is called to update predictor history when
84// a BTB entry is invalid or not found.
85}
86
87
88bool
89LocalBP::lookup(Addr branch_addr, void * &bp_history)
90{
91    bool taken;
92    uint8_t counter_val;
93    unsigned local_predictor_idx = getLocalIndex(branch_addr);
94
95    DPRINTF(Fetch, "Looking up index %#x\n",
96            local_predictor_idx);
97
98    counter_val = localCtrs[local_predictor_idx].read();
99
100    DPRINTF(Fetch, "prediction is %i.\n",
101            (int)counter_val);
102
103    taken = getPrediction(counter_val);
104
105#if 0
106    // Speculative update.
107    if (taken) {
108        DPRINTF(Fetch, "Branch updated as taken.\n");
109        localCtrs[local_predictor_idx].increment();
110    } else {
111        DPRINTF(Fetch, "Branch updated as not taken.\n");
112        localCtrs[local_predictor_idx].decrement();
113    }
114#endif
115
116    return taken;
117}
118
119void
120LocalBP::update(Addr branch_addr, bool taken, void *bp_history, bool squashed)
121{
122    assert(bp_history == NULL);
123    unsigned local_predictor_idx;
124
125    // Update the local predictor.
126    local_predictor_idx = getLocalIndex(branch_addr);
127
128    DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
129
130    if (taken) {
131        DPRINTF(Fetch, "Branch updated as taken.\n");
132        localCtrs[local_predictor_idx].increment();
133    } else {
134        DPRINTF(Fetch, "Branch updated as not taken.\n");
135        localCtrs[local_predictor_idx].decrement();
136    }
137}
138
139inline
140bool
141LocalBP::getPrediction(uint8_t &count)
142{
143    // Get the MSB of the count
144    return (count >> (localCtrBits - 1));
145}
146
147inline
148unsigned
149LocalBP::getLocalIndex(Addr &branch_addr)
150{
151    return (branch_addr >> instShiftAmt) & indexMask;
152}
153
154void
155LocalBP::uncondBranch(Addr pc, void *&bp_history)
156{
157}
158
159LocalBP*
160LocalBPParams::create()
161{
162    return new LocalBP(this);
163}
164